Tuning of the stability and energy levels of singlet exciton fission relevant excited states of pentacenes by site-specific substitution
Graphical abstract
Introduction
Singlet exciton fission (SF), a spin-allowed process, is suggested to be able to increase the photoelectric transfer efficiency of conventional photovoltaic cells by producing two triplet excitons with one photo excitation [1], [2]. The pioneer studies by Michl and co-workers have suggested that the most important energy level matching condition of SF for an isolated molecule is that the energy of singlet excited state (E(S1)) must be equal or greater than the twice of the energy of the triplet excited state (2E(T1)) [1], [3]. In order to obtain an efficient SF, this process must be fast enough to compete effectively with other radiative and nonradiative deactivation pathways, therefore, an exoergic value for ΔESF (E(S1) − 2E(T1)) is desirable. However, excessive exoergicity for SF would waste much energy and then loss of solar cell efficiency [4], so isoergicity is more favorable.
One of the most important issues that needs to be overcome is that very few materials can carry out SF efficiently. Pentacene has been regarded as the prototypical molecule among molecules that undergo SF due to its high triplet state quantum yield [5], [6], large carrier mobility [7], [8], [9], [10], and long exciton diffusion length [11], [12]. However, its poor stability towards light and oxygen, which is common for polycyclic aromatic hydrocarbons (PAHs), hindered its research in solutions and applications in organic solar cells [13], [14]. Two mechanisms have been proposed for the photooxidation of PAHs in air. They are named as electron transfer mechanism and singlet oxygen sensitization mechanism (Fig. 1S) [15]. Maliakal’s research on pentacene and 6,13-bis(triisopropylsilylethynyl)pentacene revealed that low triplet state energy of PAHs could prevent singlet oxygen sensitization [16]. The rate of electron transfer from the photo-excited PAHs to the triplet oxygen could be reduced by lower the (LUMO) energy levels, and then the photooxidation is finally hindered [16]. Miller and co-workers have studied the photooxidative resistances of a series of substituted pentacenes and proposed that singlet oxygen sensitization could be reduced by lowering the energy of the highest occupied molecular orbital (HOMO) [17]. Based on these previous investigations, low HOMO and LUMO energies and triplet excited state energies are both favorable for improving the photostability of PAHs though the degradation mechanism is complicated and some details are unknown yet [15], [16], [17], [18]. Therefore, introduction of substituents or heteroatoms to PAHs to change the energies of HOMO and LUMO and the energy of triplet excited state is an useful strategy to change the photostability [19], [20], [21], [22].
Some substituents, such as aryl [23], triisopropylsilylethynyl(TIPS) [24], halogens [20], [25], cyano group [26] and so forth, have been successfully introduced to pentacene, and the stability and solubility have been improved. Many specific modified pentacene derivatives exhibited good electronic and optical properties [27], [28]. By altering the substituents, excited-state energy levels can also be tuned [15], [20] and thus the value of ΔESF can be changed accordingly. However, there is no systematical research, which take the effects of substituents on the stability and SF ability of pentacene into consideration simultaneously, have been reported so far. In the present work, we have systematically investigated the substituent effects on the stability and SF ability of a group(series) of pentacene compounds. Try to find out the best substitution patterns which can endorse the pentacene compounds with favorable SF ability as well as good stability.
Section snippets
Computational details
The ground state geometric structures were optimized at B3LYP/6-31G** level by density functional theory (DFT), which has been widely employed in geometry optimization for aromatic compounds previously [16], [20], [29]. It is worth mentioning that the double-ζ quality AO basis set has been proved to be able to provide accurate description for molecular structures over the past decades [16], [20], [30], [31]. The vibrational frequencies were also calculated at the same level to ensure that all
Results and discussion
Pentacene ring has four different substitution positions, namely α, β, γ and δ, which can be divided into two groups, central-substitution (α, β) and end-substitution (γ, δ) (Fig. 1). The former two substitution positions normally have larger steric hindrance due to the presence of hydrogen atoms at the nearby six-member rings, whereas the steric hindrance of the later two substitution positions are small due to the hydrogen atoms nearby extend away from these positions. The introduction of
Conclusions
The effects of different substituent on the stability as well as the energy of SF relevant excited states are systemically investigated. The introduction of electron withdrawing groups can stabilize the pentacenes by lowering the energy levels of HOMO and LUMO. Most of the electron withdrawing groups studied do not change the ΔESF significantly except CN and NO2. Probably due to the strong conjugation electron withdrawing property of CN and NO2, they can reduce ΔESF significantly. Therefore,
Acknowledgments
We thank the Natural Science Foundation of China (with Grant Nos. 91233108 and 21173136) and the National Basic Research Program of China (973 Program: 2012CB93280) for the financial support.
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